I'm gonna' chose one for each (finished) series:
TOS: season 1
Hard to describe - this season is basically what the whole "Star Trek" thing is about, it opened the whole universe for me. Probably also my pick for "best Trek season. period."
TNG: Season 6
A bit of an outlier. Yes, season 3 & 4 are also super great, and season 6 starts and ends on weaker two-parters. But I just love most of the episodes in this season so much, it's just overall great quality.
DS9: season 6
I was never a fan of how the Dominion War was depicted as essentially WW2 in Space. But IMO this season is the very highpoint of the Dominion conflict, and is basically what DS9 as a whole is all about, with a great arc in the beginning, and most episodes being related to the main story.
VOY: Season 4
IMO one of the best Trek sesons, period. So many great two-parters, single episodes, and the arrival of Seven of Nine really changes things up, and makes her & Janeway one of Trek's great duos.
ENT: season 3
By far the best season-arc in Trek. There's some clunkers in the beginning, but this is IMO how modern Trek should be like - there's an overarching (very Trekkian!) main plot, big mysteries, a new enemy that is thoroughly explored, and lots of great character arcs for all main characters that are organically interwoven with the main story. And all while still keeping the "story-of-the-week"-approach of Trek.